Kazunari Hashiguchi

Establishment of human cell lines lacking mitochondrial DNA.

Mitochondria have their own genome, and mitochondrial DNA (mtDNA) encodes 2 ribosomal RNAs, 22 transfer RNAs, and 13 polypeptides that function in oxidative phosphorylation (OXPHOS). mtDNA mutations lead to dysfunction of OXPHOS, resulting in cell death and/or compromised cellular activity. Cell lines lacking mtDNA (termed rho(0) cells) are very effective tools for studying the consequences of mtDNA mutations. rho(0)cell lines have been used widely to investigate relationships between mtDNA mutation, mitochondrial function, and a variety of cellular processes. In this chapter, we summarize the yeast and animal rho(0) cell lines that have been studied. We provide simple protocols for the generation of human rho(0) cells by exposure to ethidium bromide and PCR verification of their rho(0) status.

Characterization of 2-hydroxyadenine DNA glycosylase activity of Escherichia coli MutY protein

Purpose : 2-Hydroxyadenine (2-ohA) is an oxidation product of adenine generated in DNA by ionizing radiation and various chemical oxidants. 2-ohA has mutational potential comparable to that of 8-oxoguanine in bacteria and mammalian cells. Recent studies have shown that 2-ohA is removed from DNA by a human MutY homolog, MYH protein, in vitro. On the other hand, the repair mechanisms for 2-ohA in Escherichia coli are not yet understood. Materials and methods : Gel shift assays were used to assess the binding activity of E. coli full-length MutY protein and its N-terminal (residues 1-226) domain (M25) to 2-ohA/G-, 2-ohA/A-, 2-ohA/C- and 2-ohA/T-containing 24-mer oligonucleotides. Furthermore, whether these proteins specifically cleave 2-ohA-containing duplex oligonucleotides was examined. Results : The purified MutY and M25 proteins had similar binding affinities to 2-ohA/G-, 2-ohA/A- and 2-ohA/C-containing oligonucleotides. MutY protein removed 2-ohA preferentially from 2-ohA/G mispairs. M25 protein showed the reduced catalytic activity for 2-ohA/G-containing oligonucleotides. Conclusions : E. coli MutY protein has a DNA glycosylase activity that removes 2-ohA from 2-ohA/G mispairs in DNA. The C-terminal domain is required for the removal of 2-ohA from DNA, but is not crucial for binding to 2-ohA-containing oligonucleotides.

Identification of proteins of Escherichia coli and Saccharomyces cerevisiae that specifically bind to C/C mismatches in DNA.

The pathways leading to G:C-->C:G transversions and their repair mechanisms remain uncertain. C/C and G/G mismatches arising during DNA replication are a potential source of G:C-->C:G transversions. The Escherichia coli mutHLS mismatch repair pathway efficiently corrects G/G mismatches, whereas C/C mismatches are a poor substrate. Escherichia coli must have a more specific repair pathway to correct C/C mismatches. In this study, we performed gel-shift assays to identify C/C mismatch-binding proteins in cell extracts of E. COLI: By testing heteroduplex DNA (34mers) containing C/C mismatches, two specific band shifts were generated in the gels. The band shifts were due to mismatch-specific binding of proteins present in the extracts. Cell extracts of a mutant strain defective in MutM protein did not produce a low-mobility complex. Purified MutM protein bound efficiently to the C/C mismatch-containing heteroduplex to produce the low-mobility complex. The second protein, which produced a high-mobility complex with the C/C mismatches, was purified to homogeneity, and the amino acid sequence revealed that this protein was the FabA protein of E.COLI: The high-mobility complex was not formed in cell extracts of a fabA mutant. From these results it is possible that MutM and FabA proteins are components of repair pathways for C/C mismatches in E.COLI: Furthermore, we found that Saccharomyces cerevisiae OGG1 protein, a functional homolog of E.COLI: MutM protein, could specifically bind to the C/C mismatches in DNA.

Induction of a large deletion in mitochondrial genome of mouse cells by X-ray irradiation

Abstract Large deletions and point mutations of mitochondrial DNA (mtDNA) is causally associated with mitochondrial diseases, and accumulates with age in human tissues. In order to find out the role of oxidative stress in the generation of large mtDNA deletions, using normal Balb and severe combined immunodeficiency (SCID) mouse cells, we assessed whether X-ray irradiation induces large mtDNA deletions. Cultured cells were irradiated with X-rays and assayed for a large mtDNA deletion using a PCR technique with a specific pair of primers. X-ray doses as low as 1 Gy were effective for the induction of the large mtDNA deletion (5823 bp long), which corresponds to the human 4977 bp common deletion. The breakpoints were flanked by the 5-bp long tandem repeats, 5′-TACCC-3′. A dose-dependent induction of the large mtDNA deletion was observed, the yield being higher in normal cells than in SCID cells. The fraction of large mtDNA deletion increased in the normal cells but decreased in the SCID cells within 7 days after X-ray irradiation. As the SCID cells carry a mutation in the gene encoding DNA–PKcs, the key enzyme in DNA double-strand break repair, it can be concluded that repair of DNA strand breaks may be involved in the formation of X-ray induced large mtDNA deletions.

NDX-1 protein hydrolyzes 8-oxo-7, 8-dihydrodeoxyguanosine-5'-diphosphate to sanitize oxidized nucleotides and prevent oxidative stress in Caenorhabditis elegans

8-oxo-dGTP is generated in the nucleotide pool by direct oxidation of dGTP or phosphorylation of 8-oxo-dGDP. It can be incorporated into DNA during replication, which would result in mutagenic consequences. The frequency of spontaneous mutations remains low in cells owing to the action of enzymes degrading such mutagenic substrates. Escherichia coli MutT and human MTH1 hydrolyze 8-oxo-dGTP to 8-oxo-dGMP. Human NUDT5 as well as human MTH1 hydrolyze 8-oxo-dGDP to 8-oxo-dGMP. These enzymes prevent mutations caused by misincorporation of 8-oxo-dGTP into DNA. In this study, we identified a novel MutT homolog (NDX-1) of Caenorhabditis elegans that hydrolyzes 8-oxo-dGDP to 8-oxo-dGMP. NDX-1 did not hydrolyze 8-oxo-dGTP, 2-hydroxy-dATP or 2-hydroxy-dADP. Expression of NDX-1 significantly reduced spontaneous A:T to C:G transversions and mitigated the sensitivity to a superoxide-generating agent, methyl viologen, in an E. coli mutT mutant. In C. elegans, RNAi of ndx-1 did not affect the lifespan of the worm. However, the sensitivity to methyl viologen and menadione bisulfite of the ndx-1-RNAi worms was enhanced compared with that of the control worms. These facts indicate that NDX-1 is involved in sanitization of 8-oxo-dGDP and plays a critical role in defense against oxidative stress in C. elegans.